congestion control techniques quality of services

Introduction: In computers network the data is the major thing that is to be transfer over the whole network. So the data should be passed in the smooth manner from one node to another node. For this rate of transferring the data among the nodes, in per unit time, matters a lot. The network has the capacity to transfer the data to some extent. But if the data comes to the network, is beyond to the capacity of the network then there comes the extra burden on the network. It tends to the traffic in the network and congestion occurs in the network. Congestion in the network is major issue that is concerned with the quality of the services. Improving one means the improving another. Adopting the better technique to improve quality of services is related to the congestion avoidance.  

   Data Traffic:  Congestion control and quality of services takes the data traffic in to account and to avoid it to be happened in the network. Peak data rate is the maximum data rate of the traffic in the network. The average data rate in the network is the number of bits sent per unit of time. The maximum bandwidth is used by the network is needed for the traffic without changing the data flow. Data flow traffic profile can be: constant bit data rate, variable bit rate, or bursty. In constant bit rate the average data rate is same as the peak data rate of the network. Whereas, in variable bit rate traffic profile, average data rate differs from the peak data rate in the network.   

   Congestion: Congestion is the main issue in the packet switching network where the router has the queues at the each interface of it. If number of packets sent to a network is greater than of its capacity of handling the number of packets then congestion may occur.  At each interface of router there are two queues that are used for storing data. Input data is stored in input queue and then it is processed for finding the next interface using routing table and then storing into output queue of respective interface. For congestion control the rate of packet arrival in input queue and packet departure rate from the respective output queue should be balances with packet processing rate of by the router. That is the load is the issue that is concerned with the delay as well as the throughput of the network. When the load on network goes higher than of its capacity then delay increases sharply and throughput of network decreases. For this we use the various congestion control techniques that are used for the congestion avoidance as well as for congestion removal.

Congestion control techniques are as:

             

   Open-loop Congestion Control: Open-loop policies prevent congestion before it happens. In this, source or destination avoids congestion of not to be happened in the network. These policies are as:

    Retransmission Policy: Retransmission of the data is done when the particular data is corrupted or lost in the network. But it becomes the issue when retransmission is done again and again. So the policy for the retransmission should be designed in a way such that it could prevent congestion. For this, TCP uses the retransmission policy which prevents the congestion.  

   Window Policy: This is the policy in which the corrupted data is retransmitted but only the affected part of the data is retransmitted not the entire copy of the data. This is done to avoid the retransmission of duplicated data which was not corrupted. For example, in the web browser if the timer for the packet times out then retransmission or reload is done. In that case Selective Repeat policy loads only the lost packets, which is time efficient. On other side G-Back-N policy reload all packets which set to duplicity of packets and consume time.

   Acknowledgement Policy:  The number of acknowledgements made by the receiver to the sender also effects the congestion. There should a policy that could manage the acknowledgements whenever it is needed. More the acknowledgements put more load on the network because the acknowledgement is also a data packet.

   Discarding Policy:  In this the router discards the packets in the network. Only the less sensitive packets are discarded by the router when the congestion seems to happen. This doesn’t affect the quality of data as only less sensitive packets are discarded.

   Admission Control: In this policy, router will only admit that data which will not lead to the congestion. Resource requirement for the data flow is checked before admitting into network. If there is the possibility of congestion that data is not taken by the router to avoid congestion.

   Close-loop Congestion Control: It controls the congestion after it happens. It uses the different techniques to control the congestion. These are as:

   Backpressure: It is the node to node congestion control that is started with a node and propagates in the direction of sender. Congested node stops receiving the data from its upstream node, then, next node, stops receiving data from its upstream node and so on until sender is not intimated.  Diagram shows the backpressure technique:

    Here the third node is congested and stops responding the data from second node and inform it to slow down its speed. Second node also stops receiving data from first node and so on until source is not intimated for slow down its speed. This is used only in virtual-circuit, X.25 network where the knowledge or information is there of upstream node. Hence it is not used in datagram network the router has not information of upstream node.

Choke Packet: The congested node informs the sender directly by sending a packet called choke packet. The intermediate node does not take any action; only the sender is informed to slow down its speed. Diagram shows the choke packet technique:

   The congested node third directly informs the source to slow down its speed by sending a choke packet. The intermediate nodes are not disturbed.

Implicit signaling:  Source itself checks for the congestion in the network. It makes a guess for the congestion. For example if acknowledgements are not proper source assume that network is somewhere congested and slow down its speed.

Explicit signaling:  In this the congested node sends a signal to the source or destination. In this signal is sent rather than a packet. It can be in forward or backward direction. In forward signaling, a bit can be sent in the direction of congestion i.e. to the sender. Then sender slows down its speed of transferring the packets across the network.

   In backward signaling, bit is sent opposite to the direction of congestion i.e. to the receiver. Then receiver slows down the acknowledgements to control the congestion.

    Different protocols use the different congestion control technique to control the congestion.

 Explicit signaling is better technique to control the direction because it controls the congestion, wherever it is needed, in the same or opposite direction, where the packets are required in the network. In this limited data rate can also be provided and indicates how many packets a source can send.

Quality of services: Quality of services means the services provided by network are ample as per the requirements considered by the user. Quality of services of a network is dependent of the following characteristics.

·         Reliability: Data transferring should be reliable in the network. If the network is not reliable, there may be the loss of the packets or the acknowledgment.

·         Delay: Delay is another issue in the electronic communication. There is the need of less delay in the network while transferring the packets.

·         Jitter: Jitter is the variation in the delay in the packets that belongs to same flow of data. The minimum kind of constant delay is not the big issue, whereas varying the delay times of the data become a problem in the data communication.

·         Bandwidth: Bandwidth is also concerned with the quality of services. It is dependent upon the application that how much bandwidth it would use for data transferring.

Many techniques are used to improve the quality of services. Commonly used quality of services techniques are given on next page.

Techniques to improve QoS:

·         Scheduling

·         Traffic Shaping

·         Resource Réservations

·         Admission Control

Scheduling: In a network, packets are accumulated and queued into memory buffers of routers and switches. Different techniques are used for scheduling the packets. The most common way to arrange packets is called first-in-first-out (FIFO), but various methods may be used to prioritize packets or ensure. The techniques of scheduling are as:

·         First-in-first-out (FIFO)

·         Priority Queuing (PQ)

·         Weighted Fair Queuing (WFQ)

FIFO: The FIFO discipline is the basic first-in, first-out queuing method. It is very simple: the first packet in the queue is the first packet that is served. In FIFO, all packets are treated in the same way: they are placed in a single queue and are served in the same order they were placed. When the queue becomes full, congestion occurs and the new incoming packets are dropped. Packets from different flows arrive at the router, which multiplexes them into the same FIFO queue in the same arrival.

  

PQ: PQ is the priority queuing, in which the packets assigned to a priority class. This technique uses multiple queues. Queues are serviced with different levels of priority and queues with higher priority are serviced first. The flows are the different interfaces to the router. Packets are placed in one of the queues according to their classification. Packets are scheduled from a certain queue only if the higher priority queues are empty. Packets are scheduled in FIFO order within each priority queue. In case of congestion, packets are dropped from lower priority queues. In this case, three priority queues are used. Firstly, packets from the highest priority queue are served in FIFO order.

        

 The main benefit of PQ is that packets of different classes can be managed using different queues and, consequently, a certain class of traffic can be handled in different way than another one. If the amount of highest priority traffic is excessive then the lower priority queue may not get any service until the highest priority traffic is served completely. During this while, the queues allocated to lower priority traffic may overflow. As a result, the lower priority traffic may experience a large delay or, in the worst case, or having resource starvation.

Weighted Fair Queuing (WFQ): In this technique, packets are assigned as same as priority queuing but for each queue a weight is assigned.  Higher priority queue gets large weight. Suppose for three queues weight is 3, 2, 1 respectively then three packets are chosen form highest priority queue, two from medium priority queue, and one from low priority queue for processing. This technique prevents the resource starvation for the packets. WFQ has a complex algorithm requiring as per service class state and iterative checking of all states for each packet arrival and departure.

Now, as for as, scheduling is concerned best method is weight fair queuing. Even the complex algorithm is used in it but this method allows the each flow to use the resources and burst flow of data doesn’t effects the other data flow.

Traffic Shaping: Traffic shaping is used to control the volume of traffic entering in the networks and the rate at which packets are transmitted. In this way, the flow of packets is smoothened according to the configured profile of traffic load on the network. Traffic shaping includes two techniques: leaky bucket and token bucket.

Leaky Bucket:  Leaky bucket algorithm shapes burst traffic into fixed-rate traffic by averaging the data rate. It may drop the packets if the bucket is full. Packets flowing into the bucket are paced out in steady way at a specified rate. Using a leaky bucket filter does not allow sending burst of packets, but only at a specified rate.

 

Token Bucket:    It allows router to accumulate credit for future in the form of tokens. In this token bucket is used, containing the tokens. For every data cell a single token is used. At every clock cycle a token is added to this bucket. If bucket is full of token a burst data can be transferred, it is empty then host or router cannot send data.

Token bucket is better for the bursty data transferring at regulated maximum speed than that of leaky bucket where fixed flow of data is there. In leaky bucket if there is no traffic for a certain period of time, the amount of unused bandwidth cannot be used for later packets but in token bucket this is achieved by using a token bucket filter. Hence token bucket is better than the leaky bucket.

Real time system example and algorithm

Introduction to Real time system: Real-time systems have been defined as: "those systems in which the correctness of the system depends not only on the logical result of the computation, but also on the time at which the results are produced". A real-time system is one that must process information and produce a response within a specified time; else risk apart the result, including failure. That is, in a system with a real-time constraint it is not having good chance to have the correct action or the correct answer after a certain deadline. Air line traffic control, automatic washing machines etc. are the examples of real time systems. It is of two types: Hard and Soft real time system.

    In hard real time system any output after the deadline should be treated as useless, so it lead to complete failure. For example, missile launching is the real time system. In a soft real-time system, degrading operations occur rarely and peak load can be tolerated. If load is high response time is reduced and hence minor delay can be there.

Now, as for as, the question is concerned, over here traffic lights are taken as real time system for description.

   Traffic lights: Traffic light is the hard real time system or intelligent system in which the time plays the major role. Time is set for the lights with the interaction of the physical world. The lights are used to reduce the delay while using the road by the vehicle.  The normal function of traffic lights requires sophisticated control and coordination to ensure that traffic moves as smoothly and safely as possible and that pedestrians are protected when they cross the roads. A variety of different control systems are used to accomplish this, ranging from simple clockwork techniques to sophisticated computerized control and coordination systems that self-adjust to minimize delay to people using the road.

   Traffic controller systems: A traffic signal is typically controlled by a controller inside a cabinet mounted on a concrete pad. Although some electro-mechanical controllers are still in use, modern traffic controllers are solid state. The cabinet typically contains a power panel, to distribute electrical power in the cabinet, a detector interface panel, to connect to loop detectors and other detectors, detector amplifiers, the controller itself, a conflict monitor unit, flash transfer relays, a police panel, to allow the police to disable the signal and other components.

                                        

                                              Computerized traffic control box

   Traffic controllers use the concept of phases, which are directions of movement grouped together. For instance, a simple intersection may have two phases: North/South, and East/West. A 4-way intersection has independent control for each direction. Traffic lights must be instructed when to change phase and they are usually coordinated so that the phase changes occur in some relationship to other nearby signals or to the press of a pedestrian button or to the action of a timer or a number of other inputs.

    The controller uses input from detectors, which are sensors that inform the controller processor whether vehicles or other road users are present, to adjust signal timing and phasing within the time limits set by the controller’s programming. Detectors can be grouped into three classes: in-pavement detectors, non-intrusive detectors, and detection for non-motorized road users.

   In-pavement detectors: These detectors are buried in or under the roadway. Inductive Detector loops are the most common type. They are sensors buried in the road to detect the presence of traffic waiting at the light, and thus can reduce the time when a green signal is given for an empty road. A timer is frequently used as a default during times of very low traffic density and as a backup in case the sensors fail. The sensor loops typically work in the same fashion as metal detectors.

   Non-intrusive detectors: It is sometimes more advantageous and cost effective to install over-roadway sensors than tampering the road and embedding inductive loops. These technologies include video image processors, sensors that use electromagnetic waves, or sound sensors to detect the presence of vehicles at the intersection waiting for the way.

   Non-motorized user detection: Non-motorized users are classified as pedestrians, bicyclists, and equestrians. Provisions for detecting these users include demand buttons and tuned detectors. Some traffic lights at pedestrian crossings, especially those away from junctions, include a button which must be pressed in order to activate the timing system. This is generally accompanied by a large display reading "wait", which lights up when the button is pressed, and off when the lights enter the red phase.

Now, describing the basic algorithm for the Traffic lights.  As the traffic lights are on the all four directions on the road. So the algorithm can be like as:

1.    Start

2.    Initializing and defining the variables and define the functions.

3.    Switch on Yellow on west direction and Red for remaining directions.  // west side is alert or ready and all other sides are restricted.

4.    Switch on Yellow on north direction and Red for remaining directions.

5.    Switch on Green on north direction and Red for remaining directions.

6.    Countdown timer is on, on the display.

7.    If Time is equal to zero.

      Goto step 8.

Else

     Wait for time to be zero.   //vehicle has to wait for the signal off

8.    Switch on Yellow on north direction and Red for remaining directions.   // north side is alert or ready

9.    Switch on Yellow on south direction and Red for remaining directions.

10. Switch on Green on south direction and Red for remaining directions. Goto step 6     // south side is free to move

11. Switch on Green on east direction and Red for remaining directions. Goto step 6

12.  Switch on Green on west direction and Red for remaining directions. Goto step 6

  This will keep on running at every time like this. Here is the flow chart representation of the traffic light algorithm.

                

   Traffic light are not only the alone part of the traffic control but there is the inter-relationship with the other phases, which gives the better understanding of the traffic control system. It is shown in the diagram below.

Traffic light control and information transmission device compromising a microprocessor on the cross road, the microprocessor further connects to a traffic light controller, an electronic display board, a video camera, a compression circuitry, an I/O interface, a traffic flow detector and connected to the central traffic control computer through the DSL (Digital Subscriber Loop). The control signals, traffic, public information or news of the central traffic control computer can go through the DLS to the microprocessor; the microprocessor can control the traffic light and display all the information on the electronic display board. The traffic flow data of the cross roads can be accessed by the traffic flow detector and the video camera and transmitted back to the central traffic control computer.

   Suggestion for required modification: The future traffic lights should be more interactive. As in the present traffic lights the countdown timer displays the numbers which are not mostly liked and may annoy someone waiting in red signal. Interaction can be done like as when signal becomes red the days headline news can be displayed or weather can be displayed simultaneously with the interactive countdown of traffic light device.